1. Field of Invention
The invention relates to the field of fixed-wing aircraft. More particularly, the invention relates to the various models of Lake amphibious aircraft, as manufactured by Aerofab, Inc. More particularly yet, the invention relates to a device for strengthening wing spars on such aircraft.
2. Description of Prior Art
Forces exerted on the wings of aircraft during landing impose stresses on the wing structures. These stresses are even greater on wing structures of amphibious aircraft during water landings because the shock-absorbing devices that are integrated into the landing gear are not available when landing on water. It has been determined that the method of wing spar attachment used in certain Lake amphibious aircraft models may result in cracks in the wing spar, specifically, in the wing spar cap and wing spar attachment bolt-holes. The wing spar serves to attach the wing to the aircraft fuselage and these cracks have the potential to cause separation of the wing from the fuselage during flight, with obvious deleterious consequences.
Due to the seriousness of a wing spar structural failure, the Federal Aviation Administration(FAA) issued an airworthiness directive (AD) for the wing spar on the Lake models of amphibious aircraft, directing that the referenced aircraft be repaired or modified within a specific timeframe in accordance with the AD. The particular problem to be solved was the elimination of the structural deficiencies of the wing attachment due to cracks initiating at a machined notch at the flange termination point of the wing-spar cap angle. One correction proposed was frequent inspection and replacement of the wing-spar cap angle upon the detection of cracks. This solution is, however, very costly and time-consuming--it being a very labor-intensive and time-consuming task to replace parts of the wing spar, with a typical cost of $40,000. An alternative to that first approach is to physically strengthen the wing spar prophylactically by, for example, adding an additional layer of metal to the vulnerable element.
In the field of aircraft manufacturing, the application of an additional layer of material, commonly called a "doubler," as a means of reinforcing a structural component is well known. For example, Cox (U.S. Pat. No. 4,984,347) describes a means of attaching a doubler to the damaged skin of an airplane as a means of reinforcing the damaged area. Welch et al. (U.S. Pat. No. 5,975,237) describes the use of a doubler for the purpose of reinforcing an acoustic panel for installation in the nacelle of a jet engine. Although both of these doubler inventions serve to strengthen aircraft elements, neither provides a solution to the specific problem at hand, which is not as straightforward as slapping more metal on the spar.
When using a doubler to modify a primary structural element, it is critical that the strength and rigidity properties of the doubler and the structural element complement each other. For example, a doubler-strap that is too rigid or has greater strength than the underlying element may itself cause stresses on the element and introduce additional sources of cracking and structural weakness. Conversely, a doubler-strap that is too flexible or has less strength than the underlying element will not provide the additional strength and reinforcement that is required. Without access to comprehensive engineering data on the components to be strengthened and on its related flight elements, it can be very difficult to determine the proper strength characteristics required in a doubler without having to carry out a lengthy testing process that may also include destructive tests and, consequently, be very costly because of the material costs.
A further difficulty in constructing a doubler-strap modification kit to solve the particular problem at hand is that there are a number of different aircraft models with wing spars that required strengthening, with dimensions of the area requiring strengthening varying with model, and to a lesser extent any individual plans of a particular model. It is desirable for obvious economic and safety reasons to have a strap that could be installed on all aircraft units requiring treatment.
Another factor that must be taken into account in developing a doubler as a means of structural reinforcement of a wing spar is the problem of corrosion. In order to serve its intended purpose, the wing-spar doubler must be resistant to any corrosion that could lead to structural weakness. This becomes a critical issue with amphibious planes, the wings of which may be expected to be regularly exposed to salt water to a degree not found in the non-amphibious planes that make up the vast majority of the world's aircraft. Salt water heightens the electro-voltaic effect that is present whenever dissimilar metals are in contact with one another.
Finally, as a safety issue, as well as an economic issue, the doubler reinforcement must be simple to install. Preferably, the doubler should be able to be installed using standard tools that are readily available at airplane maintenance facilities, and not require special skills beyond those of ordinary airplane maintenance personnel. In addition, it must be easily determinable upon a simple post-installation inspection that the doubler has been properly installed.
What is needed, therefore, is a cost-efficient effective means of strengthening the wing spars on all models of Lake aircraft. What is further needed is a modification that can be retrofitted to any model of Lake aircraft, properly and easily, with a minimum of disassembly and without causing collateral damage to other installed parts. What is yet further needed is such a modification that will provide a long-term solution to the wing spar cracking problem, that will not cause additional structural problems, and that is corrosion-resistant in a sea water environment and not subject to harmful electro-voltaic effects.